A. Technical Field
This relates to magnetic resonance imaging (MRI) apparatus and method and, more particularly, to MRI apparatus and method that can effectively select appropriate MR imaging conditions to avoid dangerous eddy currents to an imaged object generated due to changing magnetic field intensities (dB/dt) of a plurality of gradient magnetic fields used to acquire MR signals from the object being imaged.
B. Background
MRI (magnetic resonance imaging) involves magnetically exciting nuclear spins in an object located within a static magnetic field by applying radio frequency (RF) pulses at the nuclei's Larmor frequency. Image data is reconstructed based on acquired magnetic resonance (MR) signals induced by nutation of the nuclear spins.
MRI apparatus can acquire various diagnostic data, such as diagnostic anatomy data, diagnostic biochemistry data and diagnostic functional data for various body units. Thus, MRI apparatus is now used in various image diagnosis fields.
In recent years, high-speed imaging methods for MRI apparatus have been developed to analyze functional units of a brain or heart (e.g., cardiac function units). For instance, by applying a high-speed echo planar imaging (EPI) method, acquisition time for acquiring one MRI can be reduced to less than 100 milliseconds. Consequently, if the image data acquisition time is shortened sufficiently, it becomes possible to acquire various MRI images at desired time phases of a moving organ, such as a beating heart.
Parallel imaging (PI) methods also have been proposed as disclosed in Japanese Patent Application Publication No. 2004-329613. The PI method can acquire phase-encoded MR signals using a plurality of phase-encoding gradient magnetic fields during data read-out by using a multi-coil having a plurality of RF element coils. The PI method can generate image data in a short time having no wraparounds by performing an expansion process on the acquired MR signals based on sensitivity distributions of the RF element coils.
In such high-speed MR imaging methods, three gradient magnetic fields are respectively switched at high speed in order to shorten MR signal acquisition time. The three gradient magnetic fields may include a slice-selecting gradient magnetic field, a frequency-encoding gradient magnetic field and a phase-encoding gradient magnetic field that are all orthogonally disposed with respect to each other.
Usually, in accordance with a time-changing gradient magnetic field, eddy currents are induced in the imaged object (e.g., a living body) due to the time rate of change of magnetic field intensity (dB/dt). Typically, for high-speed MR imaging methods, it is necessary to switch the respective three strong gradient magnetic fields at a high speed (i.e., at a high time rate of change). Consequently, it may occur that induced high-density eddy currents exceed an allowable stimulus level for nerves in a living body.
For a cardiac magnetic resonance imaging (MRI) diagnosis, MR image data acquisitions are typically performed for an operator-selected oblique slice cross-section (planar volume) along a desired direction. Thus, MR image data is acquired along an oblique imaging plane (or double oblique imaging planes) by using three gradient magnetic fields (e.g., a slice-selection gradient field, a phase-encoding gradient field and a frequency-encoding gradient field). When the three gradient magnetic fields are respectively switched at high speed, the time rate of change in magnetic field intensity (dB/dt) for each of the three respective gradient magnetic fields is summed up to an effective total time rate of change dB/dt. Consequently, the summed up rate of change of magnetic field intensity (dB/dt) may easily generate dangerous eddy currents (e.g., that exceed an allowable stimulus level for a living body).
To prevent generation of dangerous eddy currents in a conventional MRI apparatus, the expected rate of change in magnetic field intensity (dB/dt) at a given gradient switching time is preliminarily calculated (e.g., at a preliminary stage when pulse sequence executing software based on a reference imaging condition is being set in conjunction with a pilot imaging mode, and MR signal acquisition conditions are being loaded into a prescribed unit of the MRI apparatus). When the preliminarily calculated maximum value of dB/dt exceeds a prescribed threshold value (e.g., corresponding to an allowable stimulus level), a warning from the MRI apparatus is given to an operator.
However, when the conventional warning notice is issued, it is required for an operator to somehow discover a more appropriate set of MR imaging conditions (i.e., conditions capable of restraining eddy current density below the proscribed stimulus level) by renewing (i.e., changing) reference MR imaging conditions with iterative trial and error operations. Such operations for discovering one or more appropriately adjusted reference MR imaging conditions consume time and reduce efficiency of MR imaging operations.